In order to supply an electrical load (device) with electrical energy or power, the load is generally connected to a line branch of an electrical supply power system via a switching unit (a switching device).
The switching unit, which is connected into the line branch upstream of the load, is provided for switching the electrical current for the electrical load on and off. During normal operation (rated operation), the switching unit switches the rated currents necessary for the electrical load. In particular situations, for example when switching on, the load may require significantly higher electrical powers and as a result the electric current flowing through the switching unit may be significantly higher than the rated current. The term overload current is then used. Finally, the resistance of the load may drop drastically or disappear as a result of a short circuit in the electrical load or in the lines leading to the load, and the electric current flowing through the switching unit may consequently assume very high values. These high short circuit currents lead to a high power loss in the switching unit and load and as a result very quickly to the switching unit or load being damaged or destroyed. For this reason, the switching units and loads must be protected from overload currents or short circuit currents which flow for an excessively long time. For this purpose, special protection devices are used which, in order to protect the load from excessively high currents, disconnect the line branch from the supply power system if such a critical current occurs. Fusible links, circuit breakers or power circuit breakers are usually used as protection units.
Switching units which are themselves able to withstand overloading or short circuits, and are therefore also referred to as intrinsically safe switching units, are desirable for providing protection from overload currents and short circuit currents. Such intrinsically safe switching units are intended to protect themselves and the line branch automatically so that when such switching units are used it is not possible for there to be any damage in the branch due to short circuit currents or overload currents.
In addition to the predominantly used mechanical switching units with switching contacts, electronic switching units, implemented using semiconductor components, are known for switching electrical currents. Such semiconductor components can be divided into current-controlled semiconductor components, including bipolar transistors and thyristors, on the one hand, and voltage-controlled semiconductor components such as the unipolar MOS (Metal Oxide Semiconductor) field effect transistors (MOSFETs) or the bipolar MOS-controlled thyristors (MCTs) or the MOS-controlled bipolar transistors (IGBTs), on the other hand. All the aforesaid semiconductor components can be switched only currents in one current direction (forward direction), i.e. only when there is a specific polarity of the applied operating voltage between an on state and an off state (switchable state). In its switched off state, each semiconductor component can switch off only up to a maximum off-state voltage (breakdown voltage). With relatively high off-state voltages there is a charge carrier breakdown which can quickly result in the component being destroyed.
WO 95/24055 A1 discloses an electrical switching device in which a semiconductor component with two FETs connected in an antiserial configuration and an interrupter contact on each side of the semiconductor component are connected into a line section.
The interrupter contacts are switched on or off by a triggering element which is connected in parallel with the semiconductor component. A control voltage of a control device is applied between the gate and source of the two FETs. A current sensor to which the control device is connected is arranged in the line section. The control device checks when a permissible short circuit current is reached or exceeded and then sets the control voltage for the two FETs in such a way that the permissible short circuit current is not exceeded by increasing the internal resistance of the FETs by way of the control voltage. The control device generates the control voltage using auxiliary energy (extraneous energy). The signal of the current sensor is used only for evaluating when a short circuit situation occurs, and when one does not occur.
WO 95/07571 A1 discloses an a.c. power controller with two MOSFETs which are connected in an antiserial configuration and are based on silicon carbide. Each SiC MOSFET can be driven via a separate gate-source control voltage. The gate-source voltage is set to have such a magnitude in the forward direction that there is a desired limitation of the drain-source current and in the inverse operating mode it is only so large that the internal body diodes of the MOSFETs are still currentless. As a result of the current limiting property of this circuit, it is possible to limit short circuit currents to an acceptable level and reduce them by means of gate-source voltages which are reduced so as to run appropriately. The gate-source voltages are generated using an external energy source.
DE 196 10 135 C1 discloses an electronic switching device which has two electrical connections for applying electrical operating voltages, a semiconductor component based on silicon (silicon component) and additionally a semiconductor arrangement.
The semiconductor arrangement comprises a first semiconductor region of a predefined conduction type and at least one further semiconductor region of the opposite conduction type, between which in each case a p-n type junction is formed. The semiconductor regions are each formed using a semiconductor with a breakdown field strength of at least 106 V/cm, in particular diamond, aluminum nitride (AIN), gallium nitride (GaN), indium nitride (InN) and preferably silicon carbide (SiC), in particular of the polytypes 3C, 4H and/or 6H.
At least one channel region which adjoins the p-n type junction in the first semiconductor region of the semiconductor arrangement is then electrically connected in series to the silicon component between the two terminals. The silicon component has an on state and an off state at operating voltages of a predefined polarity. The p-n type junction of the semiconductor arrangement is electrically connected between the two terminals in the off direction for the operating voltages. If the silicon component is in its off state, the depletion zone of the at least one p-n type junction chokes off the channel region in the first semiconductor region or even covers the entire channel region.
As a result, in the off state of the silicon component, the greater part of the operating voltage between the two terminals already drops across the depletion zone of the p-n type junction. Owing to the high breakdown field strength of at least 106 V/cm of the semiconductor which is provided for the semiconductor regions of the p-n type junction, the p-n type junction of the semiconductor arrangement can carry significantly higher off-state voltages than a p-n type junction which is formed in silicon and has the same charge carrier concentrations and dimensions. The breakdown field strength of silicon is, in comparison, approximately 2·105 V/cm. The silicon component therefore has to be configured only for the remaining part of the off-state voltage between the two terminals. This in turn results in a significantly reduced power loss of the silicon component in the on-state operating mode. In addition, the entire operating voltage between the two terminals is applied as an off-state voltage at the p-n type junction of the semiconductor arrangement in the other circuit branch. In the on state of the silicon component, the channel region in the first semiconductor region of the semiconductor arrangement is opened again and an electric current can then flow between the two terminals through the channel region.
A power MOSFET, preferably of the normally-off type, or even a MESFET (Metal Semiconductor Field Effect Transistor) is proposed as silicon component. The semiconductor arrangement is preferably embodied as a vertical JFET (junction Field Effect Transistor). The source of the JFET is short circuited to the drain of the silicon MOSFET. The drain of the JFET is electrically connected to the second terminal of the electronic switching device. The gate of the JFET is electrically short circuited to the first terminal of the electronic switching device and of the source of the silicon MOSFET. With such a known electronic device, which can be referred to as a hybrid power MOSFET or cascode circuit, it is possible to reach, in particular, off-state voltages of up to 5000 V and on-state currents between 5 A and 5000 A if silicon carbide (SiC) is used as the semiconductor material for the semiconductor arrangement. If a semiconductor arrangement of an IGBT-like hybrid based on silicon carbide (SiC) is combined with a silicon MOSFET in a further embodiment of the electronic device known from DE 196 10 135 C1, off-state voltages of up to 10,000 V and rated currents up to 100 A/chip can be reached.
The further publication DE 198 33 214 C1 discloses a JFET semiconductor arrangement which is constructed as a mesa structure with epitaxial layers and is preferably based on silicon carbide (SiC) as switching element. This high blocking capacity JFET semiconductor arrangement is proposed in particular for converter applications for drives of a variable rotation speed or as a.c. voltage switches of motor branches in which the switching components have to be operated in the normally-off state, i.e. are to automatically go into the off state when the current is removed. In this respect, it is proposed to connect the high blocking capacity JFET semiconductor arrangement in a cascode circuit with a low voltage MOSFET or low voltage Smart MOSFET, it being possible to manufacture the low voltage FET using known silicon technology.
Self-protection of the switching device is provided only to a limited degree without additional electronics with a secondary energy supply both in the case of the cascode circuit known from DE 196 10 135 C1 and also that described in DE 198 33 214 C1. Although significantly longer overcurrent or short circuit times may be permitted in comparison with conventional semiconductor switching elements based on silicon, the power loss in the semiconductor arrangement (the JFET) of the cascode circuit which is taken up when there is an overload or a short circuit over a relatively long time leads to thermal destruction of the protection element, and thus of the switching element, after a number of power system periods (in the case of a.c. voltage) or generally after the overload current or short circuit current has been applied for some time.
It is true that DE 196 10 135 C1 proposes to replace the silicon power MOSFET with what is referred to as a Smart power silicon MOSFET or a corresponding intelligent silicon component for switching purposes, in order to equip the electronic device with both switching functions and protection functions, for example overvoltage protection or overcurrent deactivation. However, DE 196 101 135 C1 does not give any specific information on how the connections in the Smart power silicon MOSFET have to be in order to implement the abovementioned protection functions, in particular the overcurrent deactivation, in particular as the saturation current of the JFET is in the normal ohmic operating range of the silicon MOSFET and is therefore not a detectable overcurrent for the MOSFET.
The further document DE 34 45 340 A1 discloses a single-direction or two-direction switch which is connected into a line branch a power system voltage of 220 V and 50 Hz in series with a load. In the embodiment as a single-direction switch, the switch comprises a MOSFET and a resistor (R1) which is connected in series with it and in an embodiment as a two-direction switch it comprises two MOSFETs which are connected in an anti serial configuration with a resistor (R1) connected between the two FETs. The voltage which drops across the resistor (R1) is tapped by a current limiting circuit with a further resistor and a bipolar transistor in the case of the single-direction switch and with two resistors and two bipolar transistors in the case of the two-direction switch. The current limiting circuit is connected to the gate of each MOSFET and reduces the control voltage—present between the gate and source—at the MOSFET if the voltage drop at the resistor (R1) becomes higher than 0.6 V. The current limiting circuit regulates the current through the resistor (R1) and thus through the entire switch to a value in the equilibrium state which brings about a voltage drop which corresponds to the base-emitter voltage of the bipolar transistors of the current limiting circuit. However, owing to the resistor (R1)—always connected in series—of the current limiting circuit, this known switch also has high electrical losses in the rated operating mode. The current limitation is carried out by way of the MOSFET switching elements themselves, their electrical resistance being controlled by way of the change in the gate voltage by the current limiting circuit.
The switch according to DE 34 45 340 A1 also has a short circuit protection in that two additional transistors (FET3 and FET4) are provided. One FET (FET4) forms a control switch which is connected together with a resistor (R5) between the two poles of the control voltage. The gate of this control switch FET is connected to the source of the other FET (FET3). The drain of this other FET is in turn connected via a diode (Da) to a pole of the power system voltage while its gate is connected to the positive pole of the control voltage. If the load is short circuited, the current in the switched on state rises to the limited value. When the latter is reached, the instantaneous a.c. voltage appears at the drain of the second FET (FET3) via the diode Da. The gate voltage at the FET3 is positive when the circuit is driven. The voltage level at the source of FET3 is equal to that of the gate of FET4 and is also positive so that the FET4 conducts. This has the effect of the control voltage at the gate or gates of the MOSFETs being reduced to a critical value below the thermal gate-source voltage, and the switch switches off.
However, this known circuit is in great danger in the event of a short circuit as the MOSFET (FET1) withstands short circuit currents thermally only for a few microseconds and is then destroyed and the deactivation device which is formed by the two transistors FET3 and FET4 cannot ensure such rapid deactivation. According to DE 34 45 340 A1, the intention is that both the current limitation and the short circuit protection function are to function without extraneous energy when the MOSFET switches off. However, the tapping of the voltage drop at the switching element in order to deactivate the switching element itself is not practicable as the MOSFET has to be configured with the smallest possible on-state resistance in order to minimize switching losses, and is therefore subject to considerable thermal loads until the overvoltage required for deactivation in the event of a short circuit is reached. This known switch is therefore not intrinsically safe.
The document WO 94/11937 A1 discloses a working circuit with a FET as electronic switching element and a temperature-dependent resistor, connected in series, as electronic protection element for protecting the switching element against excessively high Joule losses in the event of danger, in particular in an overload situation or short circuit situation. Owing to the associated heating occurring in the event of danger, the protection element bears the predominant part of the operating voltage dropping across the working circuit composed of the switching element and protection element. Switching off elements are also disclosed which, in the event of danger, automatically change the switching element into the switched off state using the energy contained in the operating current and the operating voltage if the operating current flowing through the working circuit or the voltage dropping across the working circuit or the protection element exceeds a predefined upper limiting value. These switching off device(s) include a circuit of a bipolar transistor which is connected to the gate of the FET switching element in order to switch it on or off, together with diodes and resistors as voltage divider.
WO 00/24105 A1 discloses a circuit with a PTC resistor in series with a MOSFET switching element or a JFET switching element. The PTC resistor is used as a protection element for the switching element in the event of danger. A transistor, together with a voltage divider, switches the MOSFET switching element off if the voltage across the PTC resistor exceeds a certain value, that is to say in the case of an error current or short circuit current. These switching off device(s) also operate automatically, that is to say without auxiliary energy.